1. Field of the Invention
The present invention relates in general to tilt detectors for optical head tilt servo devices in optical disc players and the like, and more particularly to a radial tilt detector for detecting a tilt in a radial direction of an optical disc.
2. Description of the Prior Art
An optical disc apparatus is generally adapted to project a small and narrow beam of light on an optical disc through an object lens to scan a fine mark (recoiling mark) on the disc so as to reproduce information recorded thereon. At this time, the surface of the optical disc may be tilted relative to a beam of light from an optical head due to a distortion of the disc, a surface vibration, etc. In this case, the beam of light from the optical head is incident on the surface of the optical disc to reproduce information recorded thereon, while being tilted relative to the also surface, thereby making it difficult to accurately read the recorded information.
FIG. 5 shows variations is tilt of a spot of light formed on the surface of an optical disc by a beam of light, relative to the disc surface. In particular, the lower part of FIG. 5 shows the shapes of the light spot and the upper part thereof shows light intensity distributions of the spot.
In more detail, FIG. 5b shows the shape and intensity distribution of the light spot when the light beam is perpendicular to the disc surface. In this drawing, the light spot is shown to have a symmetrical shape. FIGS. 5a and 5c show the shapes and intensity distributions of the light spot when the disc surface is tilted relative to the light beam. In each of these drawings, a coma-aberration is generated in the light spot on the disc, and the light spot has an asymmetrical shape.
For the purpose of preventing the above coma-aberration, the optical disc apparatus comprises a tilt servo device for correcting a tilt of an optical axis of the optical head to maintain the optical axis perpendicular to the disc surface. The tilt servo device includes tilt detection means for detecting the amount of a tilt of a beam of light projected from the optical head relative to the disc surface.
A tilt sensor is generally used to detect a tilt amount. This tilt sensor is conventionally installed in the tilt detection means separately from an optical system which generates a beam of light for reproduction of information recorded on the optical disc. In this construction the tilt sensor must be spaced apart from the object lens of the optical head at a certain distance in order to avoid an interference therebetween. However, the tilt sensor and the object lens of the optical head must become closer to each other to obtain a more approximate value to a tilt amount of a projected position of a light spot on the disc surface relative to the surface.
FIG. 6 is a perspective view showing an example of the construction of a head part in a conventional optical disc apparatus. A head part 102 is positioned under an optical disc 101, and an object lens 103 of an optical head and a tilt sensor 104 of tilt detection means are arranged on the upper surface of the head part 102. The tilt sensor 104 is arranged on the upper surface of the head part 102 in such a manner that a spot of light 106 formed by a beam of light 105 emitted from the object lens 103 passes therethrough and is substantially aligned therewith on a track 107 of the disc surface. Through this arrangement, the tilt sensor can approximately detect the position of the light spot 106 and the tilt amount of the disc surface.
However, in the head part with the above-mentioned construction, because the light spot and the tilt sensor are not perfectly aligned with each other, it is inevitable tact a value detected by the tilt sensor is in error. This cannot guarantee a sufficient degree of precision, for example, when a high decree of tilt correction is required according to a recording densification of the disc.
Further, because the optical head and tilt detection means are provided separately, the number of components increases, resulting in an increase in cost and a complexity in the construction of the head part.
Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a radial tilt detector which is capable of detecting a tilt in a radial direction of an optical disc at a high degree of precision and being cost-effective and simple in head part construction.
In accordance with one aspect of the present invention, the above and other objects can be accomplished by the provision of a radial tilt detector adapted to, by measuring an average velocity, excluding a variation occurring at regular periods, of a spot position on a recording surface of an optical disc at which a beam of light is condensed, in a focusing direction of the optical disc and an average velocity, excluding a variation occurring at regular periods, of the spot position in a radial direction of the optical disc calculate an angle defined by three vectors, or an average velocity vector in the focusing direction of the optical disc, an average velocity vector is the radial director of the optical disc and in a perpendicular direction to the focusing direction of the optical disc and a resultant vector of the average velocity vector in the focusing direction of the optical disc and the average velocity vector in the radial direction of the optical disc and detect a radial tilt angle on the basis of the calculated angle, the radial tilt angle being a tilt angle in the radial direction of the optical disc.
In accordance with another aspect of the present invention, there is provided a radial tilt detector for an optical disc information recording/reproduction apparatus which includes an optical head for projecting a condensed beam of light on a recording surface of a rotating optical disc and receiving the projected beam of light reflected from the recording surface of the optical disc, a focus actuator for focusing the condensed beam of light on its projected position of the recording surface of the optical disc, and drive means for outputting a drive signal to drive the focus actuator, the radial tilt detector being adapted to detect a radial tilt which is a tilt amount in a radial direction of the recording surface of the optical disc and comprising velocity detection means responsive to the drive signal from the drive means for detecting a velocity in a direction depending on an optical axis of the light beam at the position focused by the focus actuator; and radial tilt calculation processing means for calculating the radial tilt on the basis of the velocity detected by the velocity detection means and the number of rotations of the optical disc.
Preferably, the radial tilt calculation processing means may include a low pass filter for averaging the velocity detected by the velocity detection means for a predetermined period of time: multiplication means for multiplying the number of rotations of the optical disc by a track pitch which is an interval between adjacent tracks of the recording surface of the optical disc; division means for dividing the output of the low pass filter by the output of the multiplication means and outputting the divided result; and arctangent calculation means for taking, an arctangent of the output of the division means and outputting the resulting value as the radial tilt.
As an alternative, the radial tilt calculation processing means may include coefficient multiplication means instead of the arctangent calculation means, the coefficient multiplication means multiplying the output of the division means by a certain coefficient and obtaining the radial tilt as a result of the multiplication.